OK, seriously, that would drive me nuts. There are several solutions. Smiling posted a good idea. Another idea is to fasten a very shallow junction box to the bottom of the joist. It may protrude a teensy bit below the ceiling or not. The pendant shell may hide that. It may be possible for them to use a shallow box and notch the joist so the box is flush with the ceiling. I know removing material from a joist reduces the strength. You would need to check with the contractor or even better, the local inspector to see if that met code.

Below is a picture of several ceiling boxes. The shallow grey steel box at top center is the one I had in mind.

We planned to install a single recessed light over our sink but it could not be centered so we ended up with two smaller recessed lights.
Could you add a second pendant above the sink? Perhaps you can find a smaller version of your pendant and do that.

My coworker just finished her bathroom. When she noticed her bathroom pendant light was not centered relative to the mirror she went berserk, more so because of the contractor told her it was not a big deal and no one would notice.

What she did was to find a nice looking, big plate to cover the base of the light fixture when moving the lightfixture over. The result was great and save a bundle of not redoing the ceiling.

A big advantage to putting in 2 pendants to solve your problem would be not having the light directly in front of your eyes (this disturbs some folks more than others). The disadvantage is the cost of another pendant, of course.

What does the pendant look like? If it has a generous canopy that's bigger than 4.5", it should be able to slide up over a protruding junction box fastened to the beam itself.

Maybe a ceiling medallion large enough that the cord can move to come out the center of the medallion at the center of the window and the place where the cord comes out through the ceiling is also covered.
The ceiling could be cut away so the cord is flush, I think, where it runs to the center.
Does this make sense? Couldn't think how to say it simply.

You might even find one that is a decorative asset!
Something like one of these:

I never noticed that the light over my sink was not centered because we had a 8 or 10" flourescent light there. The electrician put in a pancake style box like the one above mentioned. Same situation as yours. Not sure if they notched the joist or not. They also put one in my half bath on the wall. Same problem.

There are several solutions. You can use a shallow ceiling box (which is what I just used), a saddle box, or an offset box. For those of you that want a recessed light where a joist is in the way, you can cut the joist and header it off into the adjacent joists on either side.

We had same issue. We'd planned for one recessed light above sink but ended up with 2 because of the joist dad-smack at the center point of the new window. We replaced our existing window with one 3 times as wide and because the temporary sink we were using was in the existing location, no one seemed to notice. When our contractor started to do electrical work he felt awful because he could have moved the window a few inches down since he was custom building the cabinets.

The funny thing is, once our new sink and countertops were installed, the 2 lights illuminate the large width of the area so much better than just one would have. And we put them on dimmers so we can adjust how much light we need. So it was a mistake that turned into something we are really happy with.